The present disclosure relates generally to Light-Emitting Diode (LED) lighting systems and controls; and more particularly to Alternating Current (AC) driven LED lighting systems and controls.
Light-Emitting Diodes or LEDs are increasingly being used for general lighting purposes. In one example, a group of so-called white LEDs is powered from an AC power source and the term “AC LED” is sometimes used to refer to such circuit. Concerns for AC LED include manufacture cost, power efficiency, power factor, flicker, lifespan, etc.
FIG. 1 demonstrates AC LED circuit 10 in the art, which simply has LED module 12 and current-limiting resistor 14. LED module consists of two LED strings connected in anti-parallel. AC LED circuit 10 requires neither an AC-DC converter nor a rectifier. Even though a DC voltage can be supplied, an AC voltage is typically supplied to input port 8 and directly powers AC LED circuit 10. Simplicity in structure and low-price in manufacture are two advantages AC LED circuit 10 has. Nevertheless, AC LED circuit 10 can only shine in a very narrow time period for each AC cycle time, suffering either low average luminance or high-current stress to LEDs.
FIG. 2A demonstrates AC LED circuit 15 in the art. Examples of AC LED circuit 15 can be found in U.S. Pat. No. 7,708,172. AC LED circuit 15 employs full-wave rectifier 18. A DC or AC voltage signal is received on input port 16. A string of LEDs are grouped into LED groups 201, 202, 203, and 204. Integrated circuit 22 has nodes PIN1, PIN2, PIN3, and PIN4, connected to the cathodes of LED groups 201, 202, 203, and 204 respectively. Inside integrated circuit 22 are ground switches SG1, SG2, SG3, and SG4, together with controller 24. When the voltage on input port 16 increases, controller 24 can switch ground switches SG1, SG2, SG3, and SG4, to possibly light on more LEDs. Operations of integrated circuit 22 have been exemplified in U.S. Pat. No. 7,708,172 and are omitted here for brevity.
FIG. 2B demonstrates AC LED circuit 30 in the art, whose example can be found in U.S. Pat. No. 8,299,724. Different from integrated circuit 22 in FIG. 2A, integrated circuit 34 in FIG. 2B has an addition node PIN0. Integrated circuit 34 further employs bypass switches SP1, SP2, SP3, and SP4, each selectively providing a bypass current path for driving current to detour a corresponding LED group. For example, when controller 32 turns on bypass switches SP1, nodes PIN0 and PIN1 are shorted together and LED group 201 darkens because no driving current flows through LED group 201.
FIG. 3 illustrates the waveforms of signals when input port 16 in FIG. 2A or 2B is supplied with an AC voltage signal. The upmost waveform shows rectified voltage VREC, which, as indicated in FIGS. 2A and 2B, refers to the voltage after full-wave rectifier 18 and upon LED group 201. The second waveform shows active LED count, meaning the number of LEDs of the LED groups that are made to light on. The four following waveforms regard with currents IG4, IG3, IG2 and IG1, respectively flowing through LED groups 204, 203, 202 and 201. Active LED count rises or descends stepwise, following the increase or decrease of rectified voltage VREC. When rectified voltage VREC increases, LED groups 201, 202, 203, and 204, according to a forward sequence, join to light on. When rectified voltage VREC decreases, LED groups 201, 202, 203, and 204, according to a backward sequence, darken. AC LED circuits 15 and 30 both enjoy simple circuit architecture and, as can be derived, good power efficiency.
There in FIG. 3 however has dark zone TDARK when no LED activates or shines. If rectified voltage VREC is a 120 Hertz signal, voltage valley, where rectified voltage VREC is about zero Volt, appears as a 120 Herts signal, causing dark zone TDARK to appear in the same frequency of 120 Hertz. Even though dark zone TDARK of 120 Hertz might not be perceivable by human eyes, it is reported that human may feel dizzy or nauseated when looking, for a long period of time, objects exposed under the lighting with the non-perceivable dark zone TDARK of 120 Hertz.